1. Field of Invention
This invention relates to metal alloy powders that are coated with a protective film during the production process and a process for making the metal alloy powders. The protective film provides for protection of the metal powders against environmental attack and reduces pyrophoric behavior of the powders.
2. Description of the Prior Art
Metal powders are produced by a number of different methods. For example, metal powders may be produced by gas atomization processes, water atomization processes, reduction metallurgy processes, carbonyl processes, or electrolytic processes. A preferred method of making fine spherical metal powders is by the gas atomization process. This process is preferred because, among other reasons, it is economical and provides for rapid production of the metal powder particles. The gas atomization process is limited only to the extent that the metal alloy composition can be melted and made to pour through a nozzle. A preferred gas atomization process is disclosed in our U.S. Pat. No. 4,619,845 entitled "Method For Generating Fine Sprays Of Molten Metal For Spray Coating And Powder Making".
Metal powders are used in a number of applications. For example, metal powders are used for thermally sprayed coatings, rapid solidification processed components, and metal injection molded parts. More specifically, component parts of diverse geometries may be fabricated by the consolidation of the powder with or without a binding agent. Parts formed with a binding agent are generally shaped in a mold at low or moderate temperatures, and parts formed without a binding agent are normally formed in a mold at low temperature and then heated to an elevated temperature where the individual metal particles are diffusively welded to one another.
Despite the usefulness of fine metal powders, they are sometimes difficult to use or work with due to their high surface to volume ratio which makes them more susceptible to environmental degradation than other metals such as bulk alloys of the same composition. This limitation manifests itself in that such metal powders are subject to environmental attack, for example, oxidation and corrosion. Additionally, some such metal powders tend to exhibit a pyrophoric behavior which presents danger in their manufacture, transportation, handling, and storage.
The prior art discloses various attempts and means for protecting metal powders against oxidation, corrosion, and spontaneous ignition. For example, it is known to make alloy powders more stable by producing thin oxide coatings on them by various methods. Specifically, it is known to produce oxide films on reactive metal powders atomized in an inert gas by slowly bleeding air or oxygen gas into the atomizer and the powder collection vessel. However, this process requires exacting slow bleeding rates to prevent temperature rise during the initial oxidation to avoid rapid and, in some cases, even catastrophic oxidation. Additionally, it is known that aluminum alloy powders with a thin protective oxide coating may be obtained by atomizing them in a reducing gas such as flue gas. This latter manner of production appears limited to aluminum alloys due to the physical and chemical properties of aluminum as the hydrogen and carbon in the flue gas can have undesirable effects with other metals.
Additionally, U.S. Pat. No. 4,170,466 discloses a water atomization process for producing fine metal particles of copper alloys with reduced levels of oxide and decreased danger of explosion of the hydrogen gas generated by oxidation of the metal by water. Water atomization is not a preferred manner of producing fine metal powders and is different from the gas atomization process in that a significantly enhanced oxygen content is usually obtained in water atomized powders. In addition, water atomized powders are generally non-spherical and thereby provide poor powder flowability and an elevated total surface area that impedes outgassing. Further, the process disclosed in this patent is not useful for alloys other than copper because the small silicon additives are not effective in preventing rapid oxidation of more reactive alloys.
Additionally, U.S. Pat. Nos. 4,240,831; 4,331,478; and 4,350,529 disclose corrosion-resistant stainless steel powders made by water or gas atomization to produce a powder with corrosion resistance. However, these latter patents do not disclose or want the formation of a protective film as in the present invention.
U.S. Pat. No. 4,187,084 discloses ferromagnetic abrasive materials and a method of making such materials by a carbonyl process. Other patents known to applicant disclosing various manners of making metal powders are U.S. Pat. Nos. 2,656,595; 3,892,600; 4,383,852; 4,572,844; 4,578,115; 4,810,284; and 4,833,040.
None of the above prior art provides for a fine, spherical metal alloy powder coated with a protective film during a gas atomization production process which can be made in a rapid and economic manner. As discussed hereafter, providing a protective film to metal alloy powders during the gas atomization production process is a novel improvement in the field of metal alloy powders.